Several studies have suggested genetics have a stronger influence on brain development among children from more affluent households. Some scientists have suggested the link between wealth and genetic influence is especially strong in the United States.

But new analysis of birth and school records suggests the influence of nature and nurture -- genes and environment -- are the same for everyone, regardless of socioeconomic status.

"While children from higher socio-economic status backgrounds have much better cognitive outcomes on average than those from lower socio-economic status households, genetics appear to matter just as much for both groups," lead study author David Figlio, economist at Northwestern University, said in a news release. "Genes matter. Environment matters. But we find no evidence that the two interact."

The new analysis was made possible by data tracking the educational performance of siblings and twins in Florida.

The conclusions drawn from the data are similar to the conclusions from previous research on correlations between birthweight and educational performance. Figlio confirmed that heavier babies tend to perform better on math and reading tests between the third and eighth grades, but found the relationship between weight and educational performance was the same for everyone.

"It's definitely still true that, from the point of view of test scores, you'd rather be a tiny baby from a wealthy family than a big baby from a poor family," Figlio said. "But birth weight matters, and it matters for everyone. It seems the same effect is at play here."

Figlio believes part of the appeal of previous conclusions was the simplicity it offered. Researchers are still struggling to understand how cognitive development is influenced by nature and nurture. The research suggesting genes play a more important role among wealthier kids suggested a breakthrough in that scientific struggle.

"Being able to say that 'genes' matter more for this group versus that group is appealing partly for its simplicity," said Jeremy Freese, researcher at Stanford. "We suspect the truth is more complicated: Some genes may matter more in richer families and other genes may matter in poorer families. There's no overall characterization."